When loading or unloading a truck parked at a loading dock, it is generally a safe practice to help restrain the truck from accidentally moving too far away from the dock. This is often accomplished by a hook-style vehicle restraint that engages what is often referred to in the industry as a truck's ICC bar (Interstate Commerce Commission bar) or RIG (Rear Impact Guard). An ICC bar or RIG comprises a bar or beam that extends horizontally across the rear of a truck, below the truck bed. Its primary purpose is to help prevent an automobile from under-riding the truck in a rear-end collision. A RIG, however, also provides a convenient structure for a hook-style restraint to reach up in front of the bar to obstruct the bar's movement away from the dock. To release the truck and prepare for the next one to enter, many restraints descend below the bar to a preparatory position.
Although the horizontal bar of a RIG is fairly standardized, the bar's supporting structure can vary significantly. In some cases, the supporting structure can interfere with the operation of the restraint. Some supporting structures can make it difficult for a vehicle restraint to sense the location of the bar and determine whether the bar is properly restrained.